The invention is in the field of electronic reproduction technology and is directed to a method and to an apparatus for the conversion of color values of a color negative with a negative/positive reversal function in the optoelectronic scanning of color negatives in devices and systems for electronic image processing.
Electronic image processing is composed essentially of the steps of image input, image processing and image output.
In the image input, for example with a color image scanner, three analog color value signals (R,G,B) are acquired by trichromatic as well as pixel-by-pixel and line-by-line optoelectronic scanning of a color original to be reproduced, whereby each color value triad (R,G,B) represents the color parts xe2x80x9credxe2x80x9d (R), xe2x80x9cgreen (G) and xe2x80x9cbluexe2x80x9d (B) of a pixel scanned in the color original. The analog color value signals are converted into digital color values and stored for the following image processing.
In the image processing, the color values (R,G,B) are usually first converted into color separation values (C,M,Y,K) according to the laws of subtractive color mixing, these color separation values (C,M,Y,K) being a criterion for the dosing of the inks xe2x80x9ccyanxe2x80x9d (C), xe2x80x9cmagentaxe2x80x9d (M), xe2x80x9cyellowxe2x80x9d (y) and xe2x80x9cblackxe2x80x9d (K) or, respectively, for the raster percentages used in the later printing process. At the same time, local or selective color corrections can also be undertaken under visual control on a color monitor in the image processing, with the goal of improving the color image reproduction or undertaking editorial color changes.
After the image processing, the image output occurs with a suitable output unit, for example an exposer or recorder, by pixel-by-pixel exposure of the color image to be reproduced on a recording material, from which the printing forms for the multicolor printing of the color image are then produced.
The color originals to be scanned are usually color reversal films (slides) or color negatives. A color reversal film can be directly viewed after being developed, since it supplies a chromatically correct image of the original. The color values (Rp, Gp, Bp) generated in the scanning of the color reversal film can therefore be directly employed for the color image evaluation of a color reversal film on a color monitor or for the drive of the color monitor after a color correction. A color negative, by contrast, does not supply a chromatically correct image of the original after being developed. It is only possible to view the photographed subject in correct colors after a copying of the color negative onto a special positive paper. A negative/positive reversal function must therefore be determined first for the color image evaluation of a color negative on a color monitor, and the color values (Rn, Gn, Bn) acquired in the scanning of the color negative must be converted into color values (Rp, Gp, Bp) representing the corresponding positive image on the basis of the identified negative/positive reversal function (Rp, Gp, Bn)=f(Rn, Gn, Bp) for the drive of the color monitor.
A method for determining a negative/positive reversal function is already known from the publication, H. Lang, Farbmetrik und Farbfernsehen, R. Oldenbourg Verlag, Munich, Vienna, 1978, ISBN 3-486-20661-3, Chapter 23, xe2x80x9cElektronische Farbumkehr und Farbkorrektur, pages 414 through 427.
A color negative film comprises three individual film layers, namely a blue-sensitive yellow layer, a green-sensitive magenta layer and a red-sensitive cyan layer. Each film layer is characterized by a color density curve Dr=f(Er), Dg=f(Eg) and Db=f(Eb), these indicating the relationship between the light quantities (Er, Eg, Eb) acting on the color negative film and the color densities (Dr, Dg, Db) respectively achieved in the individual film layers. In a color negative film, there is a linear relationship in a broad range between the influencing light quantities (Er, Eg, Eb) and the densities (Dr, Dg, Db) achieved after developing the film, since the slopes (xcex3n) of the film density curves, i.e. the gradation, are nearly linear in the usable value range. The slopes (xcex3n) are also positive, so that a great light quantity corresponds to a high density in the film layer and, thus, to a low degree of transmission. As a result of the color masking of the color negative film, with which the color falsifications caused by the secondary color densities in the film layers are corrected, the three color density curves Dr=f(Er), Dg=f(Eg) and Db=f(Eb) do not lie on top of one another, so that the density values (Dr, Dg, Db) respectively exhibit an offset value (xcex2r, xcex2g, xcex2b), as a result whereofxe2x80x94for example in the exposure of a gray scale valuexe2x80x94three different color density values (Dr, Dg, Db) arise.
In the known method for the conversion of color values of a color negative according to a negative/positive reversal function (Rp, Gp, Bp)=f(Rn, Gn, Bn), the color values (Rn, Gn, Bn) acquired, for example, by optoelectronic scanning with a color scanner are first logarithmized, intensified by a gain factor (xcex1r, xcex1g, xcex1b), are then inverted with the explained parameters (xcex3n, xcex2r, xcex2g, xcex2b) of the color density curves, and are subsequently in turn delogarithmized in order to obtain the color values (Rp, Gp, Bp). These conversion steps ensue combined according to the following approximate relationships:
Rp=xcex1r*Rnxe2x88x92xcex3n+xcex2r
Gp=xcex1g*Gnxe2x88x92xcex3n+xcex2g
Bp=xcex1b*Bnxe2x88x92xcex3n+xcex2b
In the known procedure, these relationships are realized by simple electrical circuits. The parameters (xcex1, xcex2, xcex3) of the color density curves are thereby varied by manually actuatable controllers, whereby the offset values (xcex2) are often ignored and the slopes (xcex3) of the color density curves identically prescribed for all three color parts. As a result thereof, the color reversal is ultimately controlled only via the gain factors (xcex1), whereby the gain factors are often interactively determined at the color monitor or by light measurement in an original.
GB-A-2191955 already discloses a method for the conversion of color values of a color negative wherein color values (R,G,B) are first logarithmized into color density signals, are digitized into color density data and intermediately stored. After the intermediate storing, the color density data are then color-corrected with a first look-up table (LUT) and are subsequently modified according to a negative/positive reversal function stored in a second look-up table. The modified color density data are converted into analog color density values and displayed on a color monitor. The acquisition of a negative/positive reversal function on the basis on the image luminance value and on the basis of the image depth value of a color negative, which are determined by an image analysis of the color negative as well as on the basis of calculated and defined density regions of the color negative, is not disclosed.
JP-A-6304092 is directed to a color image processing means in which the image luminance value and the image depth value of a color image are determined from the color signals (R,G,B) in order to acquire an optimum gradation curve therefrom. The acquisition of a negative/positive reversal function on the basis of the image luminance value and of the image depth value of a color negative, which are determined by an image analysis of the color negative as well as on the basis of calculated and defined density regions of the color negative, is not disclosed.
DE-A-3729188 discloses a color film inspection device for the display of color images on a color monitor, whereby color density data are acquired, the are subjected to a color and gradation correction, and the corrected color density values are supplied to a color monitor. The relationships between image luminance values, image depth values, density regions and a negative/positive reversal function resulting therefrom are not disclosed.
The traditional methods for the conversion of the color values of color negatives according to a negative/positive reversal function and for the determination of the negative/positive reversal function are based essentially on manual settings of color values with simplified parameters, so that no optimum color value conversion and, thus, reproduction quality is achieved. The results in the color value conversion can in fact be improved by subsequent color corrections in the following image processing, for example by a means color cast correction or a gradation correction, however this procedure is involved.
The invention is therefore based on the object of improving a method and a unit for the conversion of the color values of color negatives with a negative/positive reversal function such that a high precision is achieved in the color reversal for the purpose of achieving a good reproduction quality, and wherein the method will sequence automatically in order to relieve the operator of routine manual jobs.
According to the method and apparatus of the present invention for conversion of color values of a color negative with a positive/negative reversal function, color values are acquired for three color components by point-by-point and line-by-line, opto-electronic scanning of a colored negative. These color values are stored. A brightest color value as an image luminance value and a darkest color, value as an image depth value of the color negative are determined separately from the color values for each color component. Density ranges are determined for each color component by difference formation between corresponding image luminance values and image depth values of the color negative. The density range is defined by difference formation between a predetermined density value for image luminance and a predetermined density value for image depth. A positive/negative reversal function for conversion of the color values of the color negative into color values of a corresponding, chromatically correct color positive is determined from the three density ranges of the color negative and the defined density range. The functionally corresponding color values of the color positive are calculated for a value range of the color values of the color negative with the positive/negative reversal function. The functionally corresponding color values are deposited in a table memory addressable by the corresponding color values. The color values of the color negative are converted with the table memory into the color values of the corresponding color positive for further processing.